Member Avatar for localp

I need to append to Map to another Map... and this is what i did ..

import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
 
public class sampl {
 
	public static void main(String[] args) {
		 Map<String,String> FileMap =new HashMap<String, String>();
		 Map<String,String> stateMap =new HashMap<String, String>();
		 
		 stateMap.put("1", "value0");
		 stateMap.put("12", "value1");
		 stateMap.put("13", "value2");
		 stateMap.put("14", "value3");
 
		 FileMap.put("value changed", "key value");
		 FileMap.putAll(stateMap);
	
	            Iterator it = FileMap.entrySet().iterator();
	            while (it.hasNext()) {
	                Map.Entry pairs = (Map.Entry)it.next();
	                System.out.println(pairs.getKey() + " = " + pairs.getValue());
	            }	            
	        }	
	}

The Out put appears as follows

1 = value0
value changed = key value
13 = value2
14 = value3
12 = value1

What i expected here is to print the out put as follows >>

value changed = key value << So this appears first
1 = value0
13 = value2
14 = value3
12 = value1


according to the code.. i am adding FileMap.put("value changed", "key value"); First and FileMap.putAll(stateMap); thereafter. So why has the order changed in the above output.

how can i edit this code to display the output as follows >>>>

value changed = key value << This should appear first , then the map that i added
1 = value0
13 = value2
14 = value3
12 = value1

note:: First of all i need to display value changed = key value , and the display the remaining of the map..

I need to display values in the order of insertion
Help !!!!

  • 2 Contributors
  • 1 Reply
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  • 6 Hours Discussion Span
  • Latest Post Latest Post by cale.macdonald

Dani AI

Generated

Short answer: HashMap does not preserve insertion order. Use LinkedHashMap if you need entries returned in the order they were added.

In your example the call put("value changed", ...) happens before putAll(...), but because you used HashMap the iteration order is unspecified and can change. suggested TreeMap — that will sort by key (not preserve insertion order), so it is not the right tool for insertion-order output.

Example change (keeps your "value changed" first):

import java.util.LinkedHashMap;
import java.util.Map;

Map<String,String> fileMap = new LinkedHashMap<>();
fileMap.put("value changed", "key value");
fileMap.putAll(stateMap);

for (Map.Entry<String,String> e : fileMap.entrySet()) {
    System.out.println(e.getKey() + " = " + e.getValue());
}

Notes and caveats:

  • If you also want a predictable order for the entries coming from stateMap, make stateMap a LinkedHashMap (or insert its keys in the desired order). Otherwise the order of the entries copied by putAll depends on stateMap's iteration order.
  • LinkedHashMap can be constructed in access-order mode (new LinkedHashMap<>(initialCapacity, loadFactor, true)) if you want recently-accessed entries moved to the end.
  • For thread safety wrap with Collections.synchronizedMap(...) or use appropriate concurrent collections.

See the Javadoc for more details: LinkedHashMap (Java SE 8) and TreeMap (Java SE 8).

Member Avatar for cale.macdonald

A HashMap is unable to do that for you. You could look at TreeMap which is a sorted map which will let you write your own comparator while inserting key values.

Cheers

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